Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract disease, leading annually to 200,000 deaths and 3 million hospitalizations of young children worldwide. Immune prophylaxis with a monoclonal antibody (mAb), Synagis(tm) (Medimmune, Inc.), was shown to be effective 15 years ago to reduce complications of infection in premature birth infants, but it has not shown efficacy as a post-infection treatment in the much larger population of full term infants. No safe and effective antiviral drugs are available and no effective vaccine has been produced to date. Accordingly, there remains a large unmet medical need. Using a proprietary technology for screening single human B cells, Trellis has cloned a native human mAb, 3D3 that overcomes the limitations of Synagis, providing unprecedented curative responses in animals post exposure. The new mAb targets a different envelope glycoprotein, one that has been implicated in sabotage of the host immune response. This antibody has an affinity of 1 pM for a highly conserved epitope. It has direct antiviral activity in vitro in the presence of complement, with potency 100-fold better than Synagis. In mouse models, 3D3 has shown potent activity as both a direct antiviral agent and as an agent to block the lung inflammation linked to clinical pathology. This dual activity is qualitatively different from Synagis, or from any of the small molecule antiviral drugs under investigation for treating RSV. As expected for a native human antibody, 3D3 showed no binding to a panel of human tissues. It has been expressed in stably transformed CHO cells at 0.3 g/L prior to selection, indicating feasibility of generating a high expressing cell line. Thus,all of the goals normally associated with Phase I SBIR research have been achieved. The goals of this Direct Phase II proposal are to develop a Master Cell Bank of expressing cells with the capacity of producing 3D3 at commercially useful levels (>2 g/L), and to develop the analytical and toxicological data needed for initiation of human testing (IND data package).